For the purpose of this text, “purified water” means generally water of greater purity than ordinary potable water available from e.g. a municipal water distribution system. Highly purified water is required e.g. for various medical purposes, such as production of pharmaceuticals, and in the production of “water for injection”, the quality of which is defined in official pharmacopoeias. Plants for producing such purified water may be designed as so-called multieffect stills, employing a series of falling film evaporators. A falling film evaporator comprises a vertical bundle of evaporation tubes enclosed into a heating jacket. Water fed into the evaporating tubes at their upper end flows down the inner surface of the tubes, at least partly evaporating and forming steam, which emerges at the lower ends of the tube bundle together with any un-vaporized water. In many devices for producing highly purified steam or water, the flow of steam after exiting the evaporation tubes makes a 180° turn and flows upwards in a rising space provided therefore while the remaining water collects at the bottom of the device.
The upward-flowing steam may, depending on the type of device, undergo various kinds of demisting and/or purification operations, whereby the separated droplets and impurities finally join the water phase at the bottom of the effect while the steam enters the heating jacket of the subsequent effect. The water phase becomes the feed water of the subsequent effect and the steam condenses, giving off its heat to cause more steam to evaporate from the feed water, and the cascade continues until the last effect is reached. The combined condensates from the heating jackets, together with the steam from the last effect, are cooled and condensed respectively, in a final heat exchanger to form the purified product water.
In this context, a reject stream is a stream of water that is permanently removed from the process. Generally, the stream is liquid water, but may be a steam-liquid mixture. A continuous process for producing pure water by evaporation must include the removal of a sufficient amount of reject water to carry away the separated impurities. The ratio of reject water to product water is an important aspect of the efficiency of the process. It follows from the material balance, that if the reject stream is insufficient, the impurities either accumulate in the device or leave with the product, which both alternatives are highly undesirable.
A plant for production of purified water should be able to work continuously and respond rapidly to fluctuations in consumption. A falling film plant basically requires a certain period to adapt to an increase in demand. As more primary heat is put in, it takes some time until the heat has propagated through the different effects and resulted in more steam, which is then condensed to pure water. If the plant is constantly operated at full effect, purified water may have to be discarded, which is not efficient in terms of energy consumption. If the plant is shut down, cooling medium is consumed, and a subsequent startup again consumes energy. In Finnish patent 79790, a method and device are described for decreasing the required amount of feed water by splitting the residual water from the final effect in a plant for the production of purified steam or water into a reject stream and a re-circulation stream, which is added to the feed water stream.